A multiquantum barrier is a kind of superlattice in which electron waves reflected at interfaces in the superlattice interfere with each other, whereby a virtual potential barrier to electrons, higher than the classical potential barrier of the semiconductors forming the superlattice, is produced.
FIG. 15 is a schematic diagram illustrating a prior art MQB structure disclosed in Japanese Published Patent Application No. Sho. 63-46788. FIG. 16 is an energy band diagram of the MQB structure of FIG. 15. In these figures, reference numeral 110a designates an AlAs layer having a thickness of 186.8 .ANG., numeral 110b designates AlAs layers each having a thickness of 28.3 .ANG., numeral 110c designates AlAs layers each having a thickness of 22.6 .ANG., numeral 110d designates AlAs layers each having a thickness of 17.0 .ANG., and numeral 111 designates GaAs layers each having a thickness of 56.5 .ANG.. The AlAs layers 110a to 110d are quantum barrier layers, and the GaAs layers 111 are quantum well layers. In FIG. 16, reference character U.sub.e, designates an effective potential barrier, and reference character U.sub.0 designates a classical potential barrier.
In this prior art, the AlAs barrier layers 110a to 110d and the GaAs well layers 111 are alternatingly arranged with slightly varying thicknesses in the AlAs barrier layers. An effective potential barrier U.sub.e that reflects an electron 121 having an energy higher than the classical potential barrier U.sub.0 than between an AlAs bulk layer and an GaAs bulk layer is produced utilizing an electron wave interference effect.
In this prior art, it is thought that the gradual reduction in the thicknesses of the barrier layers in the MQB structure provides a stepwise increase in the potential barrier. However, in fact, all electrons are reflected by the effective potential barrier at the boundary between the well layer 111 next to the thick first barrier layer 110a and the barrier layer 110b.
FIG. 17 is a graph illustrating calculated reflectivities of electrons incident on the MQB shown in FIG. 15. As shown in FIG. 17, the effective potential barrier U.sub.e produced in the prior art MQB is only 1.4 times as high as the classical potential barrier U.sub.0.
On the other hand, high laser oscillation threshold current and degradation of output characteristics at a high temperature are problems in a conventional visible light semiconductor laser diode. FIG. 18 is an energy band diagram showing a conduction band edge structure of a conventional visible light semiconductor laser diode including a Ga.sub.0.5 In.sub.0.5 P active layer and (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P upper and lower cladding layers in the vicinity of the active layer. In the figure, reference numeral 125 designates a Ga.sub.0.5 In.sub.0.5 P active layer, numeral 126 designates an n type (Al.sub.0.5 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer, and numeral 127 designates a p type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer.
The problems mentioned above are caused by overflow of injected carriers (electrons) from the active layer into the p type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer. The overflow is caused because the difference .DELTA.E.sub.c in the barrier height in the conduction band between the Ga.sub.0.5 In.sub.0.5 P active layer and the (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer is lower than 200 meV and the carrier concentration of the p type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer is only 5.times.10.sup.17 cm.sup.-3, at most.
As a method of suppressing charge carrier over-flow, an MQB structure may be interposed between the active layer and the cladding layer. In the MQB structure described above, however, since the effective potential barrier U.sub.s produced by the MQB is only 1.4 times as high as the classical potential barrier U.sub.0, even when this MQB structure is employed in the visible light laser diode, only a little improvement in characteristics is achieved.
FIG. 19 is a diagram illustrating an MQB structure applied to a visible light laser diode disclosed in Journal of Electronic Information Communication Society, December 1991, Volume J74-C-I, Number 12, pages 527-535. In the FIGURE, the MQB structure comprises, alternatingly arranged, eleven (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P barrier layers 130 and ten (Al.sub.0.7 Ga.sub.0.8).sub.0.5 In.sub.0.5 P well layers 131.
The thickness of the first barrier layer 130 in the MQB is fixed at 80 monolayers (MLs), and the other barrier layers 130 are thinner than the first barrier layer and each of them has the same thickness. In addition, all of the well layers 131 are of the same thickness. Since the first barrier layer is as thick as 80 MLs, unwanted leakage of electrons due to tunneling is prevented. In this publication, the thickness of the barrier layers other than the first barrier layer and the thickness of the well layer are respectively varied, and the reflectivity of electrons in each case is calculated. FIGS. 20(a)-20(i) show the results of the calculations. In the figures, the thicknesses of the well layer and the barrier layer are shown as {well layer, barrier layer} at the top left-hand corner of each figure. The unit of the thickness is MLs.
As can be seen from FIG. 20(d), when the thickness of the well layer is 5 MLs and the thickness of the barrier layer is 4 MLs, the height of the effective potential barrier U.sub.e is about twice as high as the height of the classical potential barrier U.sub.0.
In the described prior art MQB structure, the height of the effective potential barrier produced by the MQB is only twice as high as the height of the classical potential barrier. Therefore, even when the MQB structure is interposed between an active layer and a cladding layer of a visible light semiconductor laser diode to suppress unwanted overflow of carriers from the active layer to the cladding layer, the characteristics of the laser diode are not significantly improved.